Solar water splitting is a promising approach to convert and store solar energy in the form of stable chemical bonds. A tandem particle-suspension reactor design¹ (shown in Figure 1), comprising semiconductor particles (photocatalysts) suspended in an aqueous solution to effect Z-scheme water splitting in the presence of soluble redox shuttles, is considered here. Using a device-scale numerical model¹, we identified reactor designs and concentration of photocatalysts and redox shuttles to yield up to 3.8% solar-to-hydrogen conversion efficiency with diffusion-driven species transport. Higher energy conversion efficiencies are predicted with natural convection facilitated species mixing. In this design, each semiconductor particle is wetted by electrolyte that contains at least four chemical species that can be involved in redox reactions on the particle surface. Therefore, selective surface catalysis becomes crucial to achieve high solar-to-hydrogen conversion efficiencies. In the present study, we develop a numerical model to evaluate the transport and kinetics of photogenerated charge-carriers inside a spherical semiconductor particle and across the semiconductor–electrolyte interface. The potential distribution within the particle is obtained by solving the Poisson-Boltzmann equation self consistently with the charge-carrier transport equations. The fluxes of the majority and minority charge-carriers across the semiconductor–electrolyte interface take into account all plausible redox reactions at the interface. Modeling results elucidate the dependence of the reaction selectivity on not only the kinetic parameters, but also on variables such as irradiance, operating temperature, particle size, recombination pathways and the electrolyte electrochemical potential. Results are further interpreted to identify strategies to boost the energy conversion efficiency for Z-scheme water splitting systems.

References

(1) Chandran, R. B.; Breen, S.; Shao, Y.; Ardo, S.; Weber, A. Z. Meet. Abstr. 2016, MA2016-01 (38), 1919–1919.